As the retired major-general was led away to the vehicle draped in the trademark black and white flag, his son and wife feared the worst.

"I have been asking the families of other officers and no one knows why they were taken," his son said by phone, breaking down in tears.

In the past week, ISIL militants who overran the city of Mosul last month have rounded up between 25 and 60 senior ex-military officers and members of former dictator Saddam Hussein's banned Baath party, residents and relatives say.

The crackdown potentially signals a rift in the extremist alliance that helped secure ISIL terrorists swift victory when they rode in from the desert to capture Mosul last month.

The northern city of around 2 million people is by far the largest to fall to the ISIL group and a central part of its plans for a so-called “Islamist caliphate.”

When the terrorist group seized large swathes of Iraq at lightning speed last month, it was supported by other Sunni Muslim armed groups.

Some tribes and former loyalists of Saddam's Baath party were eager to hit back at Iraq’s Shia leaders, even if they did not share ISIL's vision of a caliphate ruled on mediaeval Islamic precepts. But now, leaders of those groups are being ordered to swear allegiance to the new self-called caliphate.

"I think (the ISIL) wants to give the message that they are the only group in the land, that people must follow them or give up their weapons," said provincial governor Atheel Nujaifi, who is in touch with residents by phone after having fled to the Kurdish-controlled city of Arbil as Mosul fell.

Shia parliamentarian Haidar Abadi said the terrorist group was taking pre-emptive action to head off potential challenges. "ISIL knows very well they can’t stay if these groups move against them. They are not giving them the opportunity."

"ISIL called on their friends who are ex-Baathists to cooperate and they did. And now ISIL is kicking them out. Some will pledge allegiance. Those they don’t believe will pledge allegiance, they will execute," he said.

ISIL terrorists

An Iraqi national intelligence officer, confirming the arrest by militants of Saddam-era officers, said the motive was: "to panic people, or as revenge, or in the event that they would cooperate with the Iraqi government".

Nujaifi, the governor, estimated that around 2,000 Mosul residents had signed up to join the ISIL as fighters since they took the city. But he said career army officers and diehard Baathists were unlikely to be won over to ISIL.

Familiar tactics

Among those Nujaifi said had been rounded up by the terrorists were General Waad Hannoush, a Special Forces commander under Saddam, and Saifeddin al-Mashhadani, a Baath Party leader featured as the three of clubs in the US Army's "Iraqi Most Wanted" playing card deck during the 2003 US-led invasion.

The governor and some residents told Reuters that they believe ISIL's bold declaration of a caliphate last week had caused local discontent, possibly prompting the group to act to head off the first stirrings of resistance.

The move echoes ISIL tactics in neighboring Syria, where the terrorist group, an offshoot of al Qaeda, entrenched itself in the rebel-held east by eliminating other opponents of President Bashar al-Assad.

Although ISIL, the tribes and veterans of Saddam's Baath party emerged as allies last month, they have a history of enmity. Many of those nostalgic for Saddam teamed up with tribes to fight against the ISIL's predecessor, Al-Qaeda in Iraq, during the US "surge" offensive in 2006-2007.

All the more reason for ISIL to act swiftly against potential rivals while its victory last month gives it momentum.

"With the wind at their backs, there’s an incentive to seek greater control over Mosul now rather than later," said Ramzy Mardini, non-resident fellow at the Washington think-tank Atlantic Council.

"They’re not going to allow other insurgent groups to operate in Mosul," he said. "They may have their sights set on consolidation and transformation of the city into the de facto capital of the caliphate."

While Mardini said ISIL is strong enough to "strike, consolidate, and push other groups out" for now, he sees the long term fate of the group in Mosul as less clear.

"It's the worst-kept secret that the other insurgent groups that represent the Sunni movement are going to eventually turn against ISIL," he said.

Mosul has long harbored members of the Baathist militant group the Naqshbandi Army, believed to be headed by Saddam's lifelong confidant Ezzat Ibrahim al-Douri - king of spades in the US deck and the highest-ranking Baathist to evade capture.

Tribesmen with far looser ties to the old regime could also pose a threat to the militants, but ISIL seems to be focusing for now on Baathists and former army officers.

Asserting ISIL ideology so far has meant issuing a "city charter" banning tobacco, drugs and alcohol and ordering women to dress modestly and stay home.

The militants have also bulldozed and blown up ancient shrines and Shia mosques in Mosul and nearby towns, home to some of Iraq's richest cultural heritage.

Over the weekend ISIL forums and a Twitter account associated with the group posted images of fiery blasts and plumes of smoke rising under white minarets and golden domes.

Most of the city's minority population, including Christians and small groups like the Shabak Shia Muslims, have fled.

The rejection of any power sharing or alternatives to its purist state fits the group's vision of absolute rule.

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